1. Field of the Invention
The present invention relates to a novel rare earth gallate based oxide ion mixed conductive substance having a perovskite type structure. This oxide ion mixed conductive substance shows a very high electron-ion mixed conductivity and is useful as an electrode (air electrode) for a fuel cell and a gas separation membrane, such as an air separation membrane.
2. Discussion of the Background
A substance which conducts both electronically charged particles (electrons or holes) and ionically charged particles (positive or negative ions) is called a mixed conductive substance. When the electronic conductivity of such a material is expressed as .sigma..sub.e and the ionic conductivity is expressed as .sigma..sub.i, a value .sigma..sub.i /((.sigma..sub.e, +.sigma..sub.i), is the ion transference number, and a value .sigma..sub.e /(.sigma..sub.e, +.sigma..sub.i), is the electron transference number. They are, respectively, the proportion of electric conduction which is ionic conduction and electronic conduction. In a typical mixed conductive substance, the ion transference number is comparable to the electron transference number (i.e., each value is about 0.5).
Mixed conductive substances are classified roughly into alkaline ion mixed conductive substances, proton mixed conductive substances and oxide ion mixed conductive substances depending on the type of ions contributing to electric conduction. A large part of the positive active materials of a cell are mixed conductive substances, particularly an alkaline ion mixed conductive substance or a proton mixed conductive substance. For example, a large part of the cathode materials of a lithium ion secondary cell is an alkaline ion mixed conductive substance. Furthermore, proton mixed conductive substances include substances which are colored by a diffusion reaction of protons, and they are used as display elements.
In contrast, excellent oxide ion mixed conductive substances in which the ionic charge carrier is an oxide ion (O.sup.2-) are scarce. It is known, for example, that CeO.sub.2, present as a solid solution with Y.sub.2 O.sub.3, Cd.sub.2 O.sub.3 or CaO, is an oxide ion conductive substance in an oxidative atmosphere, but reveals n type electronic conduction by the reduction of Ce.sup.4+ .fwdarw.Ce.sup.3+ in a reductive atmosphere; the ion transference number is reduced and the material becomes a mixed conductive substance. This material has been investigated for use as a fuel electrode (anode) of a solid oxide fuel cell (SOFC), making use of this property. However, this material suffers from the problem that the crystal lattice volume changes to a large extent depending on the atmosphere.
Materials showing a mixed conductivity in a reductive atmosphere are known, but materials showing excellent mixed conductivity in an oxidative atmosphere have so far scarcely been known. If such materials were available, they would be useful as an air electrode (cathode) of a solid oxide fuel cell.
An air electrode of a solid oxide fuel cell should be composed of materials which are chemically stable at high temperatures in an atmosphere with an oxygen partial pressure higher than about 10.sup.-15 to 10.sup.-10 atmospheric pressure and which show high electronic conduction. Metals cannot be used, and therefore perovskite type oxides exhibiting electronic conduction have been used. LaMnO.sub.3 or LaCoO.sub.3 in which alkaline earth metals, such as Sr, Ca and Mg, are doped into the A site, have mainly been used for conventional air electrodes. These materials principally show electronic conduction, and have only a very small amount of ionic conduction. Accordingly, as described later, the electrode reaction is restricted to the vicinity of the interface between the three phases: the air electrode, the electrolyte and the air; and polarization in the air electrode becomes large, causing a reduction in the output of the cell.